1. Field
The present invention relates generally to particle accelerators. More particularly, embodiments of the present invention relate to particle accelerators having multiple output energies.
2. Description of the Related Art
A particle accelerator produces charged particles having particular energies. In one common application, a particle accelerator produces a radiation beam used for medical radiation therapy. The beam may be directed toward a target area of a patient in order to destroy cells within the target area.
A conventional particle accelerator includes a particle source, an accelerator waveguide and a microwave power source. The particle source may comprise an electron gun that generates and transmits electrons to the waveguide. The waveguide also receives electromagnetic waves from the microwave power source, such as a magnetron or a klystron. The electrons are accelerated through the waveguide by oscillations of the electromagnetic waves within cavities of the waveguide.
The electromagnetic waves and the waveguide cavities are designed to optimally accelerate electrons having a particular phase. Since the electrons emitted by the electron gun may differ in phase, a prebuncher may be used to “bunch” groups of the emitted electrons at or approximately at that particular phase. The prebuncher may consist of a cavity that receives electrons from the electron gun before the electrons are received by the accelerator waveguide. The prebuncher also receives a wave from the microwave power source. Due to variations in the phases of the received electrons, electric fields created by the wave within the prebuncher cavity will accelerate some of the electrons and deaccelerate others.
The electrons exit the prebuncher cavity and pass through a drift space. The drift space does not include a substantial accelerating electric field. The electrons therefore travel through the drift space based on their respective initial velocities upon exiting the prebuncher cavity. During their travel through the drift space, the electrons that were accelerated by the prebuncher cavity overtake electrons that were deaccelerated, thereby creating bunches of electrons. The phase of each electron in a bunch is close to the phase of other electrons in the bunch.
The electron bunches are initially received by a first portion of the accelerator waveguide. This first portion is known as a buncher. The buncher prepares the electron bunches for subsequent acceleration by an accelerating portion of the waveguide. In particular, the buncher may include tapered cavity lengths and apertures so that the phase velocity and field strength of the received electromagnetic wave begin low at the input of the buncher and increase to values that are characteristic to the accelerating portion. Typically, the characteristic phase velocity is equal to the velocity of light. As a result, the electron bunches gain energy and are further bunched toward a common phase as they travel through the buncher.
The accelerating portion of the waveguide includes cavities that are designed to ensure synchrony between the electron bunches and the oscillating electromagnetic wave received from the microwave power source. More particularly, the cavities are designed so that electric currents flowing on their surfaces generate electric fields that are suitable to accelerate the electron bunches. The oscillation of these electric fields within each cavity is delayed with respect to an upstream cavity so that an electron bunch is further accelerated as it arrives at each cavity.
A particle accelerator is usually designed to operate within a small window of output particle energies. Due to the number of factors that interact during operation, a conventional particle accelerator cannot efficiently provide particle energies outside of the small window. As described above, these interacting factors include, but are not limited to: an electron current; frequency and energy of the electromagnetic wave; shape, construction and resonant frequency of the prebuncher and waveguide cavities; and desired output energy.
Some current particle accelerators attempt to efficiently output particles having widely-varying energies. One current system uses a shunt to “short out” a portion of the accelerator waveguide and to therefore reduce particle acceleration based on a desired output energy. Another accelerator includes two separate waveguide sections for accelerating electrons based on a desired output energy. Neither of these current accelerator structures is seen to provide efficient operation at substantially different output energies.